This invention relates to balanced differential output generation circuits and more particularly to systems and methods for generating balanced modulated signals with arbitrary amplitude and phase control using modulation.
Advances in technology have allowed for smaller, lower power devices. Many of these device topologies now use balanced (or differential) input drives instead of the traditional single-ended inputs and outputs. Thus, a two-port balanced device would have four single-ended connections, input+ and inputxe2x88x92, and output+ and outputxe2x88x92. It is known that for passive devices, or active devices operating in their linear region, it is sufficient to measure the individual single-ended responses from a balanced device, and combine the results mathematically to obtain the differential or balanced response. For this to work correctly, the device must behave linearly, which means the signals are small enough such that the device behavior does not change with signal level.
However, many devices are not linear throughout their range of operation. For example, an amplifier might change its bias current between large signals and small signals. For such devices, it is necessary to drive them with real-time signals that present the proper amplitude and phase relationships. These drive signals must be presented at the input ports (+ and xe2x88x92) of the device under test (DUT) with the same amplitude and 180xc2x0 of phase difference, to be a true differential signal.
For some applications, a balun (balanced to unbalanced transformer) is often used, and is placed in close proximity to the device to avoid introducing any phase offset due to connections between the device and the balun. In test equipment applications, however, it may not be possible to control the interconnections sufficiently to maintain desired balance. Further, the signaling used in many devices is of a complex modulation form and has substantial bandwidth. Baluns may distort the modulated signals over this substantial bandwidth.
The present invention is directed to a system and method which solves the problem of modulated differential drives by creating a signal source that has two outputs, at least one of which can be controlled in phase relative to the other, and both of which could be controlled in amplitude and both of which have the same modulation envelope. In one embodiment, a coherent signal is generated in a first electronic signal generator (ESG) and applied to a second ESG. The coherent signal replaces the normal input signal of the second ESG and the I and Q inputs of the second ESG control the amplitude and phase of the output signal. This output signal is applied to a third ESG replacing the normal input signal to the vector modulator. The vector modulator is controlled by the same I and Q input as is the first ESG, thereby producing an output signal which has the same modulation envelope as the output signal from the first ESG.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.